rados.h,ReplicatedPG: add CEPH_OSD_FLAG_ENFORCE_SNAPC and use on flush

We need to ensure that even with pool snaps, we use the snapc provided in order
to ensure that the clones are written back correctly.

Fixes: #7941
Signed-off-by: Samuel Just <sam.just@inktank.com>
Merge pull request #1745 from ceph/wip-7941

rados.h,ReplicatedPG: add CEPH_OSD_FLAG_ENFORCE_SNAPC and use on flush

Reviewed-by: Sage Weil <sage@inktank.com>